A. Toothed Belt and Method
The invention relates to toothed belts formed with a molded, elastomeric type body, but more particularly, the invention relates to the construction and effects of a tensile member of spirally wound cord and a method for making a toothed belt in conjunction with such a tensile member.
Toothed belts with an elastomeric type body have a plurality of substantially evenly spaced, transversely oriented teeth. A wear-resistant member, usually of fabric, is oftentimes used juxtaposed along a peripheral surface that includes belt teeth and land surfaces between successive belt teeth. A tensile member of spirally wound cord is located in the body along an equatorial plane that is spaced from the land surfaces. When fabric is used, the cord is spaced from the land surfaces by the thickness of the wear-resistant fabric and the so-located cord defines the location of an operating pitch line. A common belt manufacturing problem occurs when the operating pitch line does not coincide with a design pitch line. Various tactics, short of making new molds, are used to try to make the two pitch lines coincide.
Current practice for making cord for toothed belts involves twisting equal denier yarn ends together to form a ply and then twisting several plies together in the opposite direction to form a belt cord. For example, 2, 3, 4, 5 and 6, etc., ply cords may be made. The number of plies used in the cord construction may dramatically influence the location of an operating pitch line. For example, a two ply cord may lay in the belt with either both plies flat or with one ply located above the other. When the plies are flat, the geometric offset of the cord centerline is theoretically equal to the radius of a ply whereas when the plies are stacked atop each other, a theoretical local geometric offset is equal to the diameter of one of the plies. The same general changing of the theoretical centerline of the cord occurs with three or four ply construction. A five ply construction is usually preferred because its theoretical centerline changes very little and thus, the centerline is located at a predictable center in the cord. Six ply cord construcions, while appearing to be geometrically sound, are usually unstable because one of the plies tends to migrate toward the center of the cord giving unequal ply lengths. The preferred five ply cord construction cannot always be made because yarn of the requisite material and denier may not be commercially available. When the requisite size cord is unavailable, other techniques are used to correct the operating pitch line to coincide with the design pitch line. Thicker or thinner fabric may be used for the tooth surface wear-resistant member to raise or lower the spirally wound cord in the molded belt body. The cord may be wound at various tension levels to change its effective spiral diameter and thereby affect the location of the operating pitch line.
Altering cord twist to achieve a desired cord diameter is not a satisfactory option because the amount of cord twist is established at an optimum level between high twist for good cord fatigue life and low twist for needed tensile modulus.